OS X El Capitan Beta 3 features bug fixes as well as under-the-hood features such as improvements in stability, compatibility, and security of your Mac. Alongside OS X El Capitan Beta 3, Apple has also launched OS X Server 5.0 beta 3 for OS X Yosemite 10.10.4 and OS X OS X El Capitan 10.11 beta systems, as well as Apple Configurator 2.0 beta 3 for OS X El Capitan.
You can get OS X El Capitan Beta 3 through Software Update or via the Mac Dev Center for registered devs, while registered public beta testers are scheduled to begin receiving OS X El Capitan Betas starting this month.